1. Field of the Invention
The invention is related to non-volatile magnetic storage devices, and in particular to a hard disk drive including a current-perpendicular-to-plane (CPP) tunneling magnetoresistance (TMR) or giant magnetoresistance (GMR) read sensor with ferromagnetic amorphous buffer and polycrystalline seed layers also acting as ferromagnetic lower shields.
2. Statement of the Problem
In many non-volatile magnetic storage devices, a hard disk drive is the most extensively used to store data. The hard disk drive includes a hard disk and an assembly of write and read heads. The assembly of write and read heads is supported by a slider that is mounted on a suspension arm. When the hard disk rotates, an actuator swings the suspension arm to place the slider over selected circular data tracks on the hard disk. The suspension arm biases the slider toward the hard disk, and an air flow generated by the rotation of the hard disk causes the slider to fly on a cushion of air at a very low elevation (fly height) over the hard disk. When the slider rides on the air, the actuator moves the suspension arm to position the write and read heads over selected data tracks on the hard disk. The write and read heads write data to and read data from, respectively, data tracks on the hard disk. Processing circuitry connected to the write and read heads then operates according to a computer program to implement writing and reading functions.
In a reading process, the read head passes over magnetic transitions of a data track on the rotating hard disk, and magnetic fields emitting from the magnetic transitions modulate the resistance of a read sensor in the read head. Changes in the resistance of the read sensor are detected by a sense current passing through the read sensor, and are then converted into voltage changes that generate read signals. The resulting read signals are used to decode data encoded in the magnetic transitions of the data track.
In a typical read head, a current-perpendicular-to-plane (CPP) giant magnetoresistance (GMR) or tunneling magnetoresistance (TMR) read sensor is electrically separated by side oxide layers from longitudinal bias layers in two side regions in order to prevent a sense current from shunting into the two side regions, but is electrically connected with ferromagnetic lower and upper shields, allowing the sense current to flow through the CPP read sensor in a direction perpendicular to the sensor plane. A typical CPP TMR read sensor comprises an electrically insulating barrier layer sandwiched between the lower and upper sensor stacks. The barrier layer is formed by a nonmagnetic MgOx film having a thickness ranging from 0.4 to 1 nm. When the sense current quantum jumps across the MgOx barrier layer, changes in the resistance of the CPP TMR read sensor are detected through a TMR effect. A typical CPP GMR read sensor comprises an electrically conducting spacer layer sandwiched between lower and upper sensor stacks. The spacer layer is formed by a nonmagnetic Cu or oxygen-doped Cu (Cu—O) film having a thickness ranging from 1.6 to 4 nm. When the sense current flows across the Cu or Cu—O spacer layer, changes in the resistance of the CPP GMR read sensor are detected through a GMR effect.
The lower sensor stack of the CPP TMR read sensor typically comprises a buffer layer formed by a nonmagnetic Ta film, a seed layer formed by a nonmagnetic Ru film, a pinning layer formed by an antiferromagnetic Ir-Mn film, and a flux-closure structure. The flux closure structure comprises a keeper layer formed by a ferromagnetic Co—Fe film, an antiparallel coupling layer formed by a nonmagnetic Ru film, and a reference layer formed by a ferromagnetic Co—Fe—B film. Four fields are induced in the flux-closure structure. First, a unidirectional anisotropy field (HUA) is induced by exchange coupling between the pinning and keeper layers. Second, an antiparallel-coupling field (HAPC) is induced by antiparallel coupling between the keeper and reference layers and across the antiparallel-coupling layer. Third, a demagnetizing field (HD) is induced by the net magnetization of the keeper and reference layers. Fourth, a ferromagnetic-coupling field (HF) is induced by ferromagnetic coupling between the reference and sense layers and across the barrier layer. To ensure proper sensor operation, HUA and HAPC must be high enough to rigidly pin magnetizations of the keeper and reference layers in opposite transverse directions perpendicular to an air baring surface (ABS), while HD and HF must be small and balance with each other to orient the magnetization of a sense layer in a longitudinal direction parallel to the ABS.
The upper sensor stack of the CPP TMR read sensor typically comprises a sense layer formed by a ferromagnetic Co—Fe—B film and a cap layer formed by a nonmagnetic Ta film. Both the Co—Fe—B reference and sense layers exhibit a “soft” amorphous phase after deposition, which will be transformed into a polycrystalline phase after annealing. With this crystallization, a Co—Fe—B(001)[110]//MgOx(001)[100]//Co—Fe—B(001)[110] epitaxial relationship is developed, and thus the TMR effect is substantially enhanced.
In order for the read head to perform magnetic recording at densities beyond 400 Gb/in2, its sensor width has been progressively reduced to below 50 nm for increasing track densities, while its read gap (defined as a distance between the ferromagnetic lower and upper shields) has been progressively reduced to below 30 nm for increasing linear densities. A further reduction in the sensor width poses a stringent photolithography challenge, while a further reduction in the read gap poses an inevitable sensor miniaturization challenge.